1. Field of the Invention
This invention relates to the formation of contact and intra-ocular lenses, and more particularly to lens manufacture with computer controlled machine tools.
2. Description of the Prior Art
The manufacture of contact and intra-ocular lenses by standard turning processes is accomplished by first creating a posterior surface in a solid lens blank, and then forming a convex surface in the blank that corresponds to the outer lens surface when the finished lens is worn. The posterior surface for a contact lens is concave, while for an intra-ocular lens it may be concave, flat or convex. The process is computer controlled using current lathe equipment. The first step of this process is illustrated in FIG. 1 for a contact lens; a similar process is employed for intra-ocular lenses. A contact lens blank 2, which initially has a "hockey puck" flat cylindrical shape, is held at the end of a hollow rotatable spindle 4. A collet 6 is carried at the front end of the spindle, and has a forward facing recess 8 into which the lens blank 2 fits. The collet 8 can be contracted about its axis in a conventional manner by withdrawing it into the interior of the spindle 4, so that only the front end of the collet protrudes out from the spindle. The spindle's forward interior surface 10 is sloped and mates with a correspondingly sloped outer surface of the collet, so that withdrawing the collet in towards the spindle causes it to clamp the lens blank 2.
A concave base curve 12 is formed in the outward facing surface of the lens blank by rotating the spindle 4 about its axis and moving it horizontally (in the X direction) against an appropriate cutting tool. The cutting tool is typically diamond-tipped, and can be mounted for translation in the vertical (Y) direction in FIG. 1 as the spindle translates in the X direction. The base curve 12 is formed by varying the protrusion of the cutting tool into the lens blank such that a maximum cutting depth is reached at the center of the base curve, and the cutting depth is gradually reduced along the desired curve as the cutting tool translates in the Y direction away from the center. Alternately, the cutting tool could be mounted upon a slide on the surface of a rotatable table with the rotation of the table, movement of the slide and translation of the spindle coordinated to produce the desired curve. The collet is loosened so that the lens blank can be removed for polishing.
Next, the lens blank's center thickness T is measured and recorded, typically by writing it in ink along its edge. This is necessary to properly align the outer lens surface, formed in the succeeding step, with the inner base curve. The partially finished lens is then mounted ("blocked") with the base curve surface carried by the head of a metal or plastic mandrel or block 14. A hot wax adhesive 16 is placed over the head of the block to receive the lens blank, and fixes the parts in relation to each other as it cools. The block 14 is held by a collet 18, a set of mechanical fingers or some other suitable mechanical device that in turn is captured at the front end of the spindle 4 and protrudes outward from the spindle.
A probe 20 is brought into contact with the outward facing surface of the lens blank 2 to determine the location of this surface relative to the spindle. The computer control then coordinates a cutting tool with the spindle movement to cut the outer lens surface. Since the center thickness of the lens blank was measured after the base curve surface was cut, the lathe equipment may be programmed to remove the proper amount of material from the lens blank to form the final lens.
The lens blank 2 must be carefully centered on the block 14 so that its optical centerline coincides with that the block. Any misalignments can seriously effect the quality of the finished lens. In general, misalignments will fall into two categories. When the centerline 22 of the lens blank 2 is offset from, but parallel to, the block's centerline 24, as illustrated in FIG. 3, a "run-out" situation exits that induces "prism" in the finished lens. It is desirable that run-out be restricted to less than 20 microns. When the lens blank 2 is shifted along the front surface of the block 14 so that its axis 22 is offset from the block axis 24 along the head surface of the block, and is also at a non-zero angle to the block axis, the situation is referred to as "wobble"; it is illustrated in FIG. 4. Like prism, it is desirable that wobble be restricted to less than 20 microns. Sometimes a prescription calls for building in a certain amount of prism and/or wobble, but the same accuracy standards also apply to this case.
The lens must be carefully centered on the block so that its optical centerline coincides as closely as possible with that of the block. As mentioned above, mechanical fingers, collets or other mechanical devices are used to attain this alignment and to hold the lens relative to the block as the blocking wax cools and solidifies. Another system that has been used to properly align the lens to the block involves holding the block in the headstock of a lathe, applying the hot wax, and then sticking the lens into position. The blocking lens assembly is then rotated, and a mechanical probe is brought up to the edge of the lens to force it into alignment with the centerline of the block while the wax is still warm and pliable.
Whichever blocking technique is used, the center thickness of the lens blank must be measured prior to blocking, and entered into the computer. Since every computer entry involves the possibility of an error, this requirement tends to result in a greater number of defective lenses. The degree of run-out and wobble are also measured and entered into the computer so that corrections can be made automatically as the outer lens surface is cut; this additional data entry requirement introduces the possibility of further errors.
Drawback collets are typically used to hold the block which carries the partially formed lens blank. However, the shaft diameter of the block is subject to manufacturing tolerances. Since the closed axial position of a drawback collet is related to the diameter of the block's shank, with the collet being drawn back further as the block's shank diameter decreases, this manufacturing variation from block to block introduces an additional possibility for error, even if batches of lenses can be formed with known and consistent center thicknesses.
The entire procedure described above is slow, cumbersome and subject to error. Variations in blocking can result from mechanical tolerances, wax thickness and errors in the blocking system, and yield unusable or poor quality lenses. Furthermore, the center thickness measurement is subject to operator error, and this data may be incorrectly recorded or assigned to the wrong lens in a production run. A separate and sometimes different center thickness must be used and entered into the computer when the front surface for each successive lens is generated; this procedure is also subject to error.